1. Field of the Invention
The present invention relates to microwave mixer circuits, and more particularly to broad band microwave mixer circuits of special applicability to communication systems and radar systems which are required to process low level signals. The invention also relates to improvements in microwave integrated circuit mixer structures.
The theory of the modulation products which are present in mixers which operate in accordance with the heterodyne principle is well kown. The traditional theory stresses the importance of certain modulation products produced when the radio frequency (RF) input signal and a local oscillator (LO) signal are applied to a nonlinear element. (The RF input signal will sometimes hereinafter be referred to by the symbol f.sub.s, and the LO signal by the symbol f.sub.p. These alternate symbols are in accordance with a convention of notations commonly used in discussions of idler frequencies, as will be hereinafter described in greater depth in connection with FIG. 1 of the drawing). These modulation products include: the sum signal, LO + RF, (f.sub.-2 in the system of notations for idler theory); the intermediate frequency signal, (IF) or difference frequency LO - RF, (f.sub.0 in the system of notations for idler theory); and the image frequency 2LO - RF, (f.sub.+1 in the system of notations for idler theory). Recent theory stresses the importance of various other products of modulation. In accordance with this recent theory, if an RF signal is sufficiently small, then the resulting frequencies can be given as f.sub.n = nf.sub.p + f.sub.o ; n = -.alpha., . . . , + .alpha.; where f.sub.o = .vertline.f.sub.s - f.sub.p .vertline. is the output (IF) frequency, f.sub.s and f.sub.p being the RF and LO frequencies, respectively. In the application of the theory to the present invention, f.sub.-1 corresponds to the signal frequency, and f.sub.+1 to the image frequency. For most mixer applications f.sub.o &lt;&lt; f.sub.p, and therefore this notation has the advantage that .vertline.f.sub.n .vertline..apprxeq.f.sub.+n .apprxeq.nf.sub.p ; n = 1, . . . +.alpha.; and the magnitude of the frequency is readily identifiable by its subscript. The three frequencies at the n.sup.th level are sometimes referred to as the n.sup.th order idler frequencies.